A plurality of Fabry-Perot interferometric sensors are optically coupled in series with each other to form an ordered optical series. Each Fabry-Perot interferometric sensor has a unique signalband and a passband. Each Fabry-Perot interferometric sensor has its unique signalband within the passbands of all of the next higher ordered Fabry-Perot interferometric sensors in the optical series so that a corresponding unique fringe signal from each of the Fabry-Perot interferometric sensors is a multiplexed output from the optical series.
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6. A method comprising:
providing a double sided polished wafer;
depositing a reflector on one side of the wafer and an anti-reflection coating on the opposing side of the wafer;
selectively disposing a patterned photoresist layer on the reflector;
attaching a handle wafer to the anti-reflection coating on the opposing side of the wafer;
selectively etching through the reflector on one side of the wafer, the anti-reflection coating on the opposing side of the wafer and partially through the wafer to define a flexure of thin material remaining from material of the wafer;
removing the handle wafer;
providing a double sided polished reference plate with a reflector on one side of the reference plate and an anti-reflection coating on the opposing side of the reference plate; and
disposing the reference plate onto and opposing the wafer separated by spacers aligned with respect to the wafer and having the corresponding reflectors on the wafer and reference plate facing each other to form a Fabry-Perot interferometric sensor.
1. A method comprising:
providing a double sided polished wafer;
depositing a reflector on one side of the wafer and an anti-reflection coating on the opposing side of the wafer;
selectively disposing a patterned photoresist layer on the reflector;
attaching a handle wafer to the anti-reflection coating on the opposing side of the wafer;
selectively etching through the reflector on one side of the wafer, the anti-reflection coating on the opposing side of the wafer and the wafer to the handle wafer to define a flexure mold;
disposing a resilient polymer into the flexure mold to form elastomeric flexures;
removing the handle wafer;
providing a double sided polished reference plate with a reflector on one side of the reference plate and an anti-reflection coating on the opposing side of the reference plate; and
disposing the reference plate onto and opposing the wafer separated by spacers aligned with respect to the wafer and having the corresponding reflectors on the wafer and reference plate facing each other to form a Fabry-Perot interferometric sensor.
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This invention was made with Government support under Grant No. CMS0330470, awarded by the National Science Foundation. The Government has certain rights in this invention.
The present application is related to U.S. patent application Ser. No. 12/545,700, filed on Aug. 21, 2009, which is incorporated herein by reference and to which priority is claimed pursuant to 35 USC 120.
1. Field of the Invention
The invention relates to the field of fabrication and implementation of a high resolution fiber optic based accelerometers.
2. Description of the Prior Art
In optics, a Fabry-Pérot interferometer or etalon is typically made of a transparent plate with two reflecting surfaces, or two parallel highly reflecting mirrors. (Technically the former is an etalon and the latter is an interferometer, but the terminology is often used inconsistently.) Its transmission spectrum as a function of wavelength exhibits peaks of large transmission corresponding to resonances of the etalon. The resonance effect of the Fabry-Pérot interferometer is identical to that used in a dichroic filter. That is, dichroic filters are very thin sequential arrays of Fabry-Pérot interferometers, and are therefore characterised and designed using the same mathematics. Etalons are widely used in telecommunications, lasers and spectroscopy to control and measure the wavelengths of light. Recent advances in fabrication technique allow the creation of very precise tunable Fabry-Pérot interferometers.
The prior art has disclosed the use of coherence division multiplexing of multiple Fabry-Perot Interferometers (FPI) based sensors in a parallel array. This scheme requires a high precision scanning reference interferometric cavity. Others have disclosed a system of six optical fiber FPI strain sensors with unique cavity path lengths which demonstrated coherence division multiplexing in series. The system required the use of a laser-referenced Michelson interferometer to resolve the signals Still others have showed a frequency domain multiplexing to resolve the signal from a serial system of three fiber FPI strain sensors. Extensive data post processing applied to data acquired from a monochromator was required to extract the sense signal. It is known to use coherence division multiplexing with three FPI temperature sensors. A temperature controlled scanning Mach-Zehnder interferometer reference is employed. It is also known to use wavelength division multiplexing of two FPI pressure and temperature sensors using finer Bragg grating or arrayed waveguide gratings.
There are no prior art systems of simply multiplexed FPI-based accelerometers in transmission. The similar systems reported in the literature above represent the present state of the art of similar sensor system technology. None of the previously proposed or developed systems use wavelength dependent reflectivity to achieve sensor multiplexing. The common characteristic of the previously developed multiplexing techniques for FPI based sensors is that these techniques require relatively complex infrastructure (optical splitters and couplers, fixed and variable optical references, gratings or waveguide systems, etc) and signal processing techniques. Sealing of these systems require multiplication of the infrastructure components. The simplicity of the system presented here is preferable for many applications.
Systems have been proposed using fiber Bragg gratings (FBG). However, since such sensors are formed in the bulk of an optical fiber, it is difficult to form sensor systems involving mobile elements (especially inertial sensors) using such systems.
The illustrated embodiments include a method for the fabrication and implementation of a high resolution fiber optic based accelerometers. The creation of the optical elements in a single process contributes to the ease of manufacture. The illustrated embodiment includes a combination of elements comprising a system with low sensor signal loss and cross-talk. An extension of this system using an alternative thin film reflector technique is also uniquely disclosed. The methods disclosed are generally applicable to the field of fiber optic sensors.
The illustrated embodiment of the invention is an optical accelerometer based on Fabry-Perot Interferometers (FPI) used in transmission. It allows such sensors to be combined in a linear array. The technique allows the optical signals from each sensor to be automatically combined (multiplexed) at different wavelengths. This occurs in a way similar to the wavelength division multiplexing (WDM) technique used in optical communication systems. Using optical filters formed from multilayers of thin films deposited on the surface of the sensor, the optical properties of each sensor in the array are designed to be transparent and reflective at different light wavelengths. Light of the wavelengths of the reflective region is used by the sensor to create an optical signal in that region that change when the sensor experiences acceleration. Light in the transparent region is passed through the device for use by other devices in the series. By using the natural variation in the fabrication of the thin film filters, the reflection/transmission characteristics of each sensor are made to be of different wavelengths. This allows the sensors of the series to be formed from a single water.
No previous seismic accelerometers have been created with integrated optical signal wavelength multiplexing capabilities. The main advantages over similar sensors systems are that the present system is simpler and requires fewer optical components to achieve signal multiplexing.
The invention is proposed to be used for acceleration sensing at multiple points over a distributed area, including, but not limited to large: system or vehicle monitoring, structural health monitoring, tactical or border monitoring, etc.
More particularly, the apparatus of the illustrated embodiments comprises a plurality of Fabry-Perot interferometric sensors optically coupled in series with each other to form an ordered optical series, each Fabry-Perot interferometric sensor having a unique signalband and a passband, each Fabry-Perot interferometric sensor having its unique signalband within the passbands of all of the next higher ordered Fabry-Perot interferometric sensors in the optical series so that a corresponding unique fringe signal from each of the Fabry-Perot interferometric sensors is a multiplexed output from the optical series.
Each Fabry-Perot interferometric sensor is characterized by wavelength-dependent reflectance.
The plurality of Fabry-Perot interferometric sensors are optically coupled with each other to form a simply connected linear array along a beam of light, for example carried in a fiber optic.
In one embodiment each Fabry-Perot interferometric sensor comprises a thin film multilayer reflector. In another embodiment the thin film multilayer reflector comprises a quarter wave multilayer reflector. In still another embodiment the thin film multilayer reflector further comprises two quarter wave reflectors with an intervening half wave dielectric layer therebetween. In yet another embodiment each Fabry-Perot interferometric sensor comprises a Rugate thin film reflector.
In the illustrated embodiment the apparatus further comprises a corresponding plurality of accelerometers, each accelerometer having a reference plate and a proof mass plate resiliently coupled to the reference plate and movable with respect thereto, the reference plate and proof mass plate having optically treated surfaces to form an optical resonance cavity between each other forming a corresponding Fabry-Perot interferometric sensor, the optical resonance cavities of the plurality of accelerometers being serially optically coupled with each other to form the ordered optical series.
The illustrated embodiment of the apparatus further comprises a broadband light input optically coupled to a first one of the Fabry-Perot interferometric sensors of the ordered optical series and an optical detector optically coupled to the a last one of the Fabry-Perot interferometric sensors of the ordered optical series.
In one embodiment the apparatus comprises a corresponding plurality of seismic accelerometers, each seismic accelerometer having a reference plate and a proof mass plate resiliently coupled to the reference plate and movable with respect thereto, the reference plate and proof mass plate having optically treated surfaces to form an optical resonance cavity between each other forming a corresponding Fabry-Perot interferometric sensor, the optical resonance cavities of the plurality of accelerometers being serially optically coupled with each other to form an ordered optical series, each Fabry-Perot interferometric sensor having a unique signalband and a passband, each Fabry-Perot interferometric sensor having its unique signalband within the passbands of all of the next higher ordered Fabry-Perot interferometric sensors in the optical series so that a corresponding unique fringe signal from each of the Fabry-Perot interferometric sensors is a multiplexed output from the optical series.
The illustrated embodiments of the invention further include a method comprising the steps of: providing a double sided polished wafer; depositing a reflector on one side of the wafer and an anti-reflection coating on the opposing side of the wafer; selectively disposing a patterned photoresist layer on the reflector; attaching a handle wafer to the anti-reflection coating on the opposing side of the wafer; selectively etching through the reflector on one side of the wafer, the anti-reflection coating on the opposing side of the wafer and the wafer to the handle wafer to define a flexure mold; disposing a resilient polymer into the flexure mold to form elastomeric flexures; removing the handle wafer; providing a double sided polished reference plate with a reflector on one side of the reference plate and an anti-reflection coating on the opposing side of the reference plate; and disposing the reference plate onto and opposing the wafer separated by spacers aligned with respect to the wafer and having the corresponding reflectors on the wafer and reference plate facing each other to form a Fabry-Perot interferometric sensor.
In another embodiment the illustrated method comprises the steps of: providing a double sided polished wafer; depositing a reflector on one side of the wafer and an anti-reflection, coating on the opposing side of the wafer; selectively disposing a patterned photoresist layer on the reflector; attaching a handle wafer to the anti-reflection coating on the opposing side of the wafer; selectively etching through the reflector on one side of the wafer, the anti-reflection coating on the opposing side of the wafer and partially through the wafer to define a flexure of thin material remaining from material of the wafer; removing the handle wafer; providing a double sided polished reference plate with a reflector on one side of the reference plate and an anti-reflection coating on the opposing side of the reference plate; and disposing the reference plate onto and opposing the wafer separated by spacers aligned with respect to the wafer and having the corresponding reflectors on the wafer and reference plate facing each other to form a Fabry-Perot interferometric sensor.
In either one of the embodiments the step of depositing a reflector on one side of the wafer comprises depositing a thin film multilayer reflector on one side of the wafer. In one embodiment the step of depositing a thin film multilayer reflector on one side of the wafer comprises depositing a thin film quarter wave multilayer reflector on one side of the wafer. In another embodiment the step of depositing a thin film quarter wave multilayer reflector on one side of the wafer comprises depositing two thin film quarter wave multilayer reflectors on one side of the wafer with a half wave dielectric layer therebetween. In another embodiment the step of depositing a reflector on one side of the wafer comprises depositing a Rugate reflector on one side of the wafer.
The scope of the invention further includes a method comprising fabricating a plurality of Fabry-Perot interferometric sensors using natural variation in fabrication of the thin film filters to make unique reflection/transmission characteristics of each Fabry-Perot interferometric sensor at different wavelengths so that a plurality of Fabry-Perot interferometric sensors of the series to be formed from a single wafer.
In one embodiment each of the Fabry-Perot interferometric sensors is a seismic accelerometer with integrated optical signal wavelength multiplexing capability.
The scope of the invention includes a method of using the plurality of Fabry-Perot interferometric sensors of claim 1 for acceleration sensing at multiple points over a distributed area, including, but not limited to large system or vehicle monitoring, structural health monitoring, tactical or border monitoring.
While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of “means” or “steps” limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with a color drawing will be provided by the Office upon request and payment of the necessary fee.
The invention and its various embodiments can now be better understood by turning to the following detailed description of the preferred embodiments which are presented as illustrated examples of the invention defined in the claims. It is expressly understood that the invention as defined by the claims may be broader than the illustrated embodiments described below.
The invention is a method for creating systems of optical sensors based on Fabry-Perot Interferometers (FPI) used in transmission. The sensors are combined in a serial (in-line) array along a single optical path such as with an optical fiber. This configuration allows the sensors to be connected into sensor networks and the optical signals from each sensor to be automatically combined (multiplexed) at different wavelengths. This occurs in a way similar to the Wavelength-Division-Multiplexing (WDM) networking techniques employed in optical communication systems. In general, the creation of such systems composed of FPI-based sensors is not possible without the techniques described here, since a conventional FPI-based sensor acts as a ‘loss filter’ blocking light wavelengths needed by downstream devices. Such a sensor system has not been previously reported.
Using optical reflectors formed from multilayers of thin films deposited on the surface of the sensor, the optical properties of each sensor in the array are designed to be transparent and reflective at different light wavelengths. Light of the wavelengths of the reflective region is used by the sensor to create an optical signal that changes when the sensor experiences a change in the measurand, e.g. acceleration. Light in the transparent region is transmitted through the device for use by other devices in the series. By using the natural, predictable variation in the fabrication process of the thin film multilayers, the reflection/transmission characteristics of each sensor are made to be of different wavelengths for each device. This allows the sensors of the series to be formed from a single process simplifying the creation of the system and reducing the cost.
In the illustrated embodiments, the serialization of FPI-based sensors is demonstrated by the formation and characterization of FPI-based passive, linear accelerometers. However, the technique of the wavelength multiplexing of FPI-based sensor signals through the use of wavelength dependent reflectance of FPI-based sensors is general to all sensors that can be implemented based on transmission-type Fabry-Perot Interferometers. These sensor types include pressure sensors, chemical sensors, rotational and linear inertial sensors, etc. The described technique can be used to allow the serialization of such sensors into extended sensor networks. In addition to the integrated multiplexing capabilities of the network, the advantageous properties of such a network include:
Similar optical sensors systems exist in the form of the widely studied systems of fiber Bragg gratings (FBG). These systems have many of the same advantages. However, such systems do not have the versatility as system based on Fabry-Perot Interferometer disclosed here. The sensor elements of such FBG systems are integrated into the bulk of an optical fiber and are used principally as displacement sensors in the form of strain gages or as temperature sensors. They are not generally useful for the formation of sensitive inertial sensors as in the illustrated embodiments or other sensors based on small deformations under small loads, such as pressure or acoustic sensors.
Consider first some background information relating to the Fabry-Perot Interferometer. The Fabry-Perot Interferometer (FPI) is an elegant device that can be constructed of two parallel plates with reflective inner surfaces. When the gap between the plates is spaced on the order of a light wavelength, an optical resonance cavity is formed that defines a transmitted intensity peak, known as a interferometric fringe. All other light wavelengths are blocked. As such, such devices are frequently used as optical bandpass loss-filters. The sensitive dependence of the spectral (wavelength) placement of this fringe on the gap spacing between the plates makes such a device suitable for a variety of high-sensitivity sensor applications. However, the interferometric fringe formed from the resonance cavity is only created from light wavelengths at which the plates of the FPI are highly reflective. If the reflectance of the plates are engineered to be only strongly reflective over a limited range, then the it will only act as an optical resonance cavity over that range and can be made transparent or at least with negligible reflectivity to wavelengths outside that range. These transparent regions can be made to pass interferometric fringe signals from other similar devices in the series.
This effect is used in this work to create a system of two devices that demonstrate the FPI-based sensor serialization principle. The ideal (lossless) optical transmission response (T) of the parallel plate FPI is described by the Airy function, given for operation in air as
where RS(λ) is the reflectance of the inner surface of the plates, λ is the light wavelength and t is the gap between the plates. Conventionally, devices are designed assuming that Rs is constant over the wavelengths of intended use and Rs≠f(λ). In the illustrated embodiments, devices are specifically designed such that Rs=f(λ). In the ideal case, Rs is either high (Rs≈1) or low (Rs≈0). This forms either signal bands (SB) or a pass bands (PB) over the wavelengths ΔλSB and ΔλPB, respectively.
In the low reflectance region, Rs(ΔλPB)≈0 and the transmission T is constant (T≈1) for all wavelengths in the passband and the device is transparent to any signals of these wavelengths. In the high reflectance region, Rs(ΔλSB) approaches 1 as closely as possible and the transmission is that of a conventional FPI, such that the interferometric fringe placement is dependent on the gap between the plates and is given by
where n is an integer (n=1; 2; 3 : : : ) known as the order of the FPI and specifically identifies each fringe, and wavelength λ is the position of the interferometric fringe.
Turn now and, consider the FPI-based Sensor. From equation (2) above, relative parallel displacement between FPI plates causes a shift in the wavelength position of the transmitted fringe and the relative shift between the plates (Δt) is given by
where λi is the wavelength of the fringe before the plate shift and λ is the wavelength of the fringe after the shift. Monitoring the shift (Δλ) in the fringe wavelength is the basis of many sensors, including the ones developed in the illustrated embodiments.
Turn now and consider serialized FPI Sensors. For a linear series of FPI sensors defined in terms of their individual spectral (wavelength-dependent) transmittance (normalized transmission), (T(λ)), the total spectral transmittance of the serial sensor network (Tseries(λ)) is the simple product of the individual sensor transmittances such that
Tseries(λ)=T1(λ)×T2(λ)×T3(λ) . . . , (4)
where T1;2;3 : : : (λ) are the transmission characteristics of each sensor (measured or estimated from equation (1) above) at each wavelength over the region at which the system is intended for use.
If the signalband of one sensor is placed at the wavelengths over which the passbands of the sensors in the rest of the series fall, then the interferometric fringe signal of the sensor will be transmitted through the series. If the sensor system is designed such that all the signal bands of the individual sensors falls on the passbands of all other sensors, each sensors signal with appear in the transmitted output of the system, as shown in
Consider now thin film multilayer reflectors. Highly reflective, low-loss surfaces are commonly created from multilayer stacks of thin film dielectric materials. These structures are composed of alternating layers of high and low index of refraction material. Reflections from the interfaces between each layer combine to form regions of high and low reflection at specific wavelengths. This property is used in the illustrated embodiments to form wavelength dependent reflectors.
For example, consider first quarter wave multilayer reflectors. If the thickness of each layer are all equal to one quarter optical path length of a specified wavelength, then the surface will have a high reflectance at that wavelength. The high reflectance region will extend over a wavelength range centered at the specified wavelength and then fall off, creating a wavelength-dependent ‘notch’ reflector. The formed reflectance properties approximate the ideal wavelength dependent characteristics required by the system. However, for the reflectance region to extend over a narrow enough band of wavelengths to multiplex a number of devices, the layer stack must be able to be formed from an large number of layers of alternating index of refractions with small differences. In addition, the thickness of each layer must be closely controlled in order to maintain wavelength dependent reflectance characteristics close to the ideal characteristics shown in
Consider nest quarter, half and quarter wave multilayer filters. If two quarter wave reflectors are split by a dielectric layer of half-wave optical thickness, then the structure acts as a ‘fixed’ Fabry-Perotfilter. In the illustrated embodiments, this layer is referred to as the critical layer. In such, the quarter wave structures act as reflectors forming an optical cavity in the half wave layer. This provides a narrow, low reflectance region within the extended high-reflectance region that can be used as a WDM passband. The wavelength position of the transmitted light is dependent on the thickness of the critical layer (tc) as given by
where λc is the wavelength of the passed light and μ is the index of refraction of the central, critical layer material.
In contrast to the single quarter wave multilayer structure, this structure is used here to form reflectors in which the wavelength dependence is tied to only one dielectric layer. The index difference between each layer of the quarter wave reflectors can be made large and high reflectance can be achieved with a low number of layers. Requirement on the thickness control of each layer is relaxed and the variation in the thickness of the half wave layer allows the wavelengths of the transmitted light (acting as a signal passbands) to be varied among devices fabricated from a single wafer. However, the reflectance characteristics are inverted from that of the Quarter Wave Multilayer filters such that serialization of only two devices is achievable. It is this effect that is employed in the proof of concept of the illustrated embodiments.
Turn now to consider rugate reflectors. Rugate thin film filters have the preferential characteristics of the previous two methods without many of the trade-offs. They have optical ‘notch’ reflector characteristics similar to quarter wave multilayer reflectors, but are created from a single thin film layer with a index of refraction that varies continuously through the film thickness. Such films are generally formed by varying the chemical composition of the film during deposition. They necessarily do not have large differences in index of refraction between the maximum and minimum value. Due to this, reflective bands over narrow bandwidths suitable for the creation of serializable FPI sensors are readily achieved. Additionally, the reflectors are formed from a single film layer and as such do not require strict thickness control over each of a number of discrete layers. Rugate films thus have ideal characteristics for the formation of wavelength dependent reflectors for serializable FPI reflectors and would allow the serialized sensor network embodiments to be generalized to a large number of nodes.
Turn now to the application of a passive accelerometer 32 diagrammatically shown in
where m is the mass of the sensing element (proof mass), c is the effective damping, k is the total stiffness and s is the Laplace variable, which can be expressed as a complex frequency s=iω. Y is the base displacement, X is the proof mass displacement, Z is relative displacement between the two, and ay is the acceleration applied to the sensor (
where the angular natural frequency ωn=√k/m and the damping ratio ζ=c/(2mωn). Commonly, such sensors are designed to operate at frequencies in the ‘flat’ (gain stable with respect to frequency) response region below their natural frequency, with the response derived from equation (7) above as s→0 as
Thus, the acceleration on the devices is proportional to the relative displacement between the proof-mass and the base reference. In usual operation, the base is fixed to the item or structure that is being monitored through the sensor enclosure.
Now turn to consider the FPI-Based Accelerometer. The spectral position of an interferometric fringe created by an optical cavity between two reflective surfaces provides a high-precision pick-off (sensing) mechanism for the spacing between the proof mass and the reference of a passive accelerometer 32 as diagrammatically shown in
where λ is the wavelength of the displaced fringe under acceleration a. The acceleration can be monitored by the modulation of the transmitted optical power, as with a laser, by
where [δT/δλ]λ
Thus, the acceleration on an FPI-based accelerometer 32 can be expressed as a change in the power of a laser 38, symbolically denoted in
The simplicity of the illustrated embodiments is preferable for many applications. Furthermore, the previous schemes have been only demonstrated for slowly varying (quasi-static) measurements and it is not understood how they will behave for the dynamic signals encountered under a wider range of sensor types, especially inertial. The requirement of fiber Bragg gratings or an arrayed waveguide to provide wavelength multiplexing increases the complexity of the system. In the illustrated embodiments, the multiplexing capability is integrated into the optical structure of each sensor.
Consider now the fabrication process used in the illustrated embodiments. The fabrication processes of
An alternative process illustrated in
Consider now a passband design. The inner multilayer wavelength-dependent reflector surfaces of each sensor were designed to be quarter, half or quarter wave multilayer filters, as described above. The high index of refraction dielectric material (μH=3:7) is chosen to be amorphous or α silicon (α-Si). The low dielectric index of refraction dielectric material (μL=1:4) is chosen to be silicon dioxide (SiO2). These particular materials were chosen because they have the largest difference in index of refraction of materials commonly deposited by PECVD onto silicon substrates. This allows the highest possible reflectance with the minimum number of dielectric layers.
Ten layers were deposited on a full circular silicon wafer substrate by using alternating PECVD deposition steps. The SiO2 quarter wave layer thicknesses are 264 nm and the α-Si quarter wave layer thicknesses are 104 nm. The central half wave layer of α-Si is targeted to have a 208 nm thickness. This thin film design is intended to place a passband fringe at the 1550 nm wavelength. The chemical vapor deposition rate of dielectric material is well known to have a strong radial dependence and will vary from the center of the substrate due to limited diffusion of reactants across the surface of the substrate. Most fabrication processes are tuned to minimize this effect in order to create uniform layers across the wafer. Here this dependence is used to form layers with thickness that vary with radial position. This effect is taken advantage of here to allow the sensor components for the serial system to be fabricated from a thin film filter deposited onto a single substrate wafer. The wavelength position (λc) of the passband (ΔλPB) formed by the high transmittance fringe of the thin film filter is very sensitive to variations in the central critical layer thickness (tc), according to
Consider the signalband design. During the final step of the fabrication process, the proof mass and reference plate structures are aligned relative to each other manually before having the optical cavity gap spacing fixed with a spacer. The wavelength position of interferometric fringe is monitored during the alignment. If the wavelength (λ) of this fringe is within the high reflectance region of the reflector (the signal band ΔλSB), then the fringe will be sharp (having high interferometric finesse) and be suitable for high resolution sensing. In the current system implementation, all wavelengths outside the low reflectance, high transmittance passband fringe are available for use as the signalband. The spacing (t) is set such that the interferometric fringe created by the optical cavity between the proof mass and the reference plates is at the wavelength in the passbands of the other sensors in the serial array, as defined by equation (2), such as those shown in
We can now demonstrate the passband operation. In order to operate effectively, the optical passband of each Fabry-Perot sensor must have two characteristics. It has to:
Be of high optical transmittance (approaching unitary) such that passed signals are not attenuated significantly.
Have optical transmittance invariant to changes in the measurand (i.e. acceleration) such that passed signal are not changed (modulated) causing cross-talk between sensors.
The wavelength position of the passband fringe is only on the thickness of the central critical thin film. Accordingly, displacement between the proof mass and reference plate due to acceleration does not change the position of the passband. As previously mentioned, the high transmittance of the passband fringe is accomplished by using thin film materials with large differences in optical index of refraction.
We further demonstrate multiplexed sensing. The operation of single FPI sensors fabricated as above has been previously demonstrated by the present inventors. See M. A. Perez, E. J. Eklund, and A. M. Shkel. Designing micromachined accelerometers with interferometric detection. IEEE Sensors 2005 Conference Proceedings, September 2005. Using similar techniques, a system of two serialized devices is demonstrated. The demonstration is achieved by directing a tunable laser (HP 8168E) through two appropriate devices with wavelength shifted transmittance characteristics in series and monitoring the transmitted power via an amplified photodetector (Thorlabs PDA255). The laser output power is fixed (1000 μW) at the half-power wavelength of each devices fringe. Both samples are simultaneously excited acoustically via a voice coil in a sinusoidal fashion. As the interference fringe moves from its rest position under the load, the transmitted optical power is attenuated according to the sample's optical and mechanical characteristics according to equation (10). The wavelength of the laser is switched between the half-power point of the two devices to eliminate the need for demultiplexing the wavelength signals after transmission but the orientation of the devices is not adjusted. The acoustic input power is monitored by a small condenser microphone (Radio Shack 33i3013) is manifested as a voltage. The response of each device as a optical power modulation is also monitored as a voltage from the photodetector. For the serialized system with the serial multiplexed fringe characteristic of
Thus, it can be understood that the illustrated embodiments are directed to:
Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following invention and its various embodiments.
Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention as defined by the following claims. For example, notwithstanding the fact that the elements of a claim are set forth below in a certain combination, it must be expressly understood that the invention includes other combinations of fewer, more or different elements, which are disclosed in above even when not initially claimed in such combinations. A teaching that two elements are combined in a claimed combination is further to be understood as also allowing for a claimed combination in which the two elements are not combined with each other, but may be used alone or combined in other combinations. The excision of any disclosed element of the invention is explicitly contemplated as within the scope of the invention.
The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself.
The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims below or that a single element may be substituted for two or more elements in a claim. Although elements may be described above as acting in certain combinations and even initially claimed as such, it is to be expressly understood that one or more elements from a claimed combination can in some cases be excised from the combination and that the claimed combination may be directed to a subcombination or variation of a subcombination.
Insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of the claims. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements.
The claims are thus to be understood to include what is specifically illustrated and described above, what is conceptionally equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention.
Shkel, Andrei A., Perez, Maximillian A.
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